Circuit for pneumatic controllers

ABSTRACT

A circuit for pneumatic PID controllers including a differential pressure amplifier having positive and negative feedback paths each including a timing element and a pressure divider wherein a buffer amplifier is connected between the timing element and the control point pressure divider and/or between the timing element and the setpoint pressure divider for energy decoupling.

I United States Patent 1 1 1111 3,782,403 Bader 1 1 Jan. 1, 1974 [54] CIRCUIT FOR PNEUMATIC 3,380,465 4/1968 Rona 137/81.5 CONTROLLERS 3,395,719 8/1968 Boothe et a1. 137/815 X 3,388,713 6/1968 Bjornsen 137/815 Inventor: Horst Baden stuttgart-Fasanenhof, 3,417,769 12/1968 Bjornsen et a1 137/815 Germany 3,429,248 2/1969 Furlong l37/8l.5 X 3,495,774 2 1970 H 235 200 PF [73] Assignee: J. C. Eckardt AG., Stuttgart, aspen I Germany Primary Examiner-Alan Cohan [22] Filed: 1971 Assistant Examiner-Gerald A. Michalsky [21] Appl. No.: 171,635 Attorney-Craig, Antonelli & Hill [30] Foreign Application Priority Data [57] ABSTRACT ug Germany 20 0 169 8 A c1rcu1t for pneumatic PlD controllers including a [52] U5 CL 137/86 137/84 differential pressure amplifier having positive and neg- [51] hm'cl Gosd 16/00 ative feedback paths each including a timing element [58] Field 5 82 84 and a pressure divider wherein a buffer amplifier is 137/86 connected between the timing element and the control point pressure divider and/0r between the timing ele- [56] References Cited ment and the setpoint pressure divider for energy del' UNITED STATES PATENTS Coup mg 3,339,571 9/1967 Hatch 137/815 20 Claims, 5 Drawing Figures PATENTED 1 74 saw 1 or z INVENTOR HORST BA DER 7 ATTORNEY 1 CIRCUIT FOR PNEUMATIC CONTROLLERS This invention relates to a circuit for pneumatic PlD controllers with feedback as disclosed in my U.S. Ap-

plication Ser. No. 863,642, filed Oct. 3, 1969, now U.S. 5

Pat. No. 3,621,860, consisting of at least one amplifier fashioned as a differential pressure amplifier, the input signals of this amplifier being constituted by pressures derived with the aid of a pressure divider from the sum of the actual value and positive feedback signals, as well as the sum of the setpoint and negative feedback signals.

Controller circuits of this type make it possible to design systems of pneumatic PID controllers which do not exhibit any mechanical comparison elements, such as proportional bellows, rings, drums, disks, and the like, the movements of which must be converted into pressure values by a nozzle-baffle system.

The conventional circuit as described in application Ser. No. 863,642, now U.S. Pat. No. 3,621,860, starts with a differential pressure amplifier, the input pressure chambers of which are bounded by elastic walls (diaphragms or spring bellows). The amplifier does not w withdraw any energy from the pressure divider in the form of compressed air, i.e., the pressure dividers are not burdened by the amplifier. In order to produce the output pressure y of the controller, the amplifier withdraws compressed air from an energy source, such as the supply air pressure p In accordance with this previously disclosed circuit, the pressure dividers are connected, via the timer elements (throttle storage means), to the output pressure y of the controller, and an energy connection exists, respectively, between a pressure divider and the adjacent timer element; however, this connection can have a disadvantageous effect on the adjustability or the setting of the controller parameters X,,, T,,, and T The disadvantage resides in that the effective time behavior of the controller depends on the setting of the P-range (proportional range). Once the P-range is altered, the time behavior of the controller is likewise changed. This is the case particularly when the timing elements are set to large values. The time behavior effective at the output of the controller deviates from the values for T, and T,, adjusted at the scales of the timer elements. The objective of the present invention resides in eliminating the influence of the P-range setting on the effective time behavior of the controller and furthermore in improving the static and dynamic properties of 0 the controller. lt is also an objective to realize such properties with simple structural components.

According to this invention, the provision is made to arrange. for purposes of energy decoupling (isolation), a buffer amplifier between the timer element T and the resistor of the control point pressure divider and/or between the timer element T,. and the resistor of the rated value pressure divider.

By means of this measure, each pressure divider is isolated, from an energy viewpoint, from the vicinal timer element. The invention also makes it possible to avoid additional disadvantageous properties inherent in conventional pneumatic controllers.

One conventional design ofa pneumatic PID controller with a mechanical comparison member in the form of a proportioning bellows is provided, in the feedback path, with a pressure divider serving for setting the P- range. Between the l-volume of the l-timer element and the fixed resistor of this pressure divider, a 1:1 buffer amplifier is disposed. However, this step avoids the disadvantageous property that an adjustment of the P- range at the pressure divider is connected with a pressure jump or surge of the output signal of this controller. This controller design is equipped with a mechanical comparison means and a nozzle-baffle system; consequently, this design is based on a circuit which is considerably different from that disclosed in application Ser. No. 863,642, now U.S. Pat. No. 3,621,860, with which the present invention is associated. It is generally known that the adjustable range for the derivativeaction or rate time T,, and the integral-action or reset time T,, is limited. The reason for this is the mutual influence (inter-action) existing between the timer elements T,, and T if these timer elements are disposed in a feedback channel, which is the case in any singlechannel controller. This adjustable range is even additionally narrowed for the time parameters T, and T, by an energetic coupling of these timer elements.

According to the procedure of this invention as suggested herein, it is now made possible to avoid also the energetic coupling between the timing elements T,, and T, without additional expenditure, by providing the buffer amplifier between the setpoint pressure divider and the timer element T,., and between the timing element T, and the branch point in the feedback channel from which the feedback signal is branched off to the timing element T,,. in this arrangement, the buffer amplifier simultaneously isolates the timer element T, from the rated value pressure divider and from the timing element T It is furthermore known that also in controllers with an l-portion (PI- and PlD-controllers) in the decaying condition, i.e., time t the control deviation X,,= x w does not become zero. The modulating range yields information regarding the extent of the control deviation X,, which cannot be controlled by the controller with I-proportion. This modulating range is produced, inter alia, by the finite amplification in the forward branch of the controller, as well as by the properties (hysteresis) of the elastic walls (diaphragms and spring bellows) which depend on the magnitude of the pressure exerted thereon. The modulating range varies with the adjusted proportional range x, and the operating point of the controller. With a high amplification in the forward branch of the controller, the modulating range can be kept small; however, with an increase in the forward amplification, a reduction in the limit frequency is produced, which means a deterioration of the dynamic properties and an increase in susceptibility to oscillations.

In accordance with this invention, the additional provision is made to furthermore fashion the circuit according to the above-referenced U.S. application in such a manner that the modulating range, and thus the control deviation X, can be brought to as small a value as possible. This is attained by providing that, in at least one of the buffer amplifiers arranged in accordance with this invention, the gain v deviates from one and/or is adjustable to values around the value one (predominantly between l.0 v l .5). This arrangement is based on the consideration that the difference E, E required for producing the respective output pressure can be obtained at the input of the differential pressure amplifier from a corresponding difference between the feedback magnitudes R" and R formed from the output signal y of the controller.

For a particular controlled system, there is a control pressure y which makes the control deviation X zero. Depending on the magnitude of this control pressure y, a difference is necessary at the differential pressure amplifier which depends, inter, alia, on the amplification of the differential pressure amplifier proper, on the set P- range at the pressure dividers of the circuit, and on the properties (hysteresis and the like) of the elastic walls. This pressure difference at the differential pressure amplifier required in each case is formed, according to this invention, by a corresponding difference A R between the feedback signals R and R. A buffer amplifier is employed for forming A R, the amplification v of this amplifier being adjustable.

The features of this invention make it possible to set, by varying the amplification factor of the buffer amplifier, any controller output pressure y necessary for the con o d v a iwliaiifls 1 9, 19% .tqi l a the static properties. The features of this inventibiT-ifisb make it possible to keep the modulating range small with a relatively low gain of the control amplifier (forward gain). The limit frequency of the control amplifier is then higher, which has an advantageous effect on the dynamic properties of the controller.

A calculation shows that the control deviation X becomes zero when the buffer amplifier exhibits the gain v l+ V l, and the operating point thereof assumes the value l+)\/A V y,,. In this connection, )tx, indicates the set proportional range of the controller and V is the gain in the forward branch of the controller. The symbol y,, is the operating point of the amplifier which is thus taken into account.

A further embodiment of the invention serves to fashion the buffer amplifier likewise as a differential pressure amplifier, and to make its gain adjustable by a pressure divider in the negative feedback (as suggested in German Patent Application P 20 40 166.5).

For several cases of application for pneumatic PID controllers, it proved to be advantageous to produce the D-behavior in the actual value branch x with the aid ofa D-relay and to design the controller as a PI controller. For this conventional circuit, the invention provides a solution wherein the controller amplifier, the buffer amplifier, and the D-relay are fashioned as a differential pressure amplifier and are provided with negative feedback. (A D-relay of this arrangement is the subject matter of German Patent Application P 20 40 168.7).

A simple, spaceand cost-saving construction is attained if the differential pressure amplifiers are provided with two elastic walls (diaphragms, spring bellows) which define on one side two input pressure chambers and are in operative connection with two balls of equal diameter displaceably arranged in a bore of a constant diameter, and if, in a conventional man ner, a feed duct for the compressed air is disposed in alignment with the contact point of the balls, and at least one discharge duct is provided which terminates at a spacing of one-half the ball diameter from the feed duct into the bore. Also, an advantage proved to be the formation of structural units. Thus. the provision is made to combine the buffer amplifiers with the adjacent timing elements, and the controller amplifier with the two pressure dividers, in each case to form one structural unit. These units are disposed on a base or supporting plate provided with pressure lines. Also, the further combination of units into a single or unitary structure, with the objective of creating a compact and space-saving design for pneumatic controllers, is included in the further development of the present invention.

In summation, is to be noted that the invention makes it possible to produce several embodiments of pneumatic controllers with a small number of different structural components. The required components are balls, elastic walls (diaphragms, spring elements), and throttle resistors (capillaries, needle valves), as well as one or more housings provided with lines for the supply and discharge of compressed air.

The invention will be explained in greater detail with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic circuit diagram of this invention;

FIG. 2 shows a practical embodiment of the circuit of FIG. 1 wherein the buffer amplifiersare fashioned as differential pressure amplifiers and are combined with a timing element into one structural unit;

FIG. 3 is a schematic circuit diagram wherein a D- relay is disposed in the actual value branch;

FIG. 4 illustrates a practical embodiment of the circuit of FIG. 3; and

FIG. 5 is a schematic diagram showing the various structural units arranged on a base plate.

FIG. 1 shows the differential pressure amplifier I (control amplifier), the input pressures E and E of which are derived from between the resistors 2 and 3 of the actual value pressure divider and the resistors 4 and 5 of the rated value pressure divider. The actual value x is applied to the throttle resistor 2,'and the rated value w is applied to the resistor 4. In order to set the P-range x,,, for example, the resistors 2 and 4 are fashioned to be adjustable in the form of needle valves, and the resistors 3 and 5 are fashioned as a fixed throttle in the form of capillaries.

The output of the differential pressure amplifier l, to which a further amplifier is connected, if necessary, represents the output pressure y of the controller. The feedback values R and R are formed from the output signal y with the aid of the timing elements T, and T,, (as described in my U.S. Application Ser. No. 863.642. now U.S.Pat. No. 3,621,860).

For purposes of an energy decoupling of the signals, the buffer amplifier 6 is arranged between the timing element T, and the resistor 3 of the actual value pressure divider, and the buffer amplifier 7 is disposed between the timing element T, and the resistor 5 of the rated value pressure divider. The buffer amplifiers 6 and 7 as well as the control amplifier 1 are connected to the supply air pressure p,; According to this invention, the provision is made to fashion at least one of the buffer amplifiers in such a manner that the control deviation X,,. can be brought to small values and thus the modulating range of the controller is improved, even in case the gain of the forward branch of the controller is smaller than in conventional designs.

The effect of this feature of the present invention will be explained with the aid of a calculation. For purposes of simplification, it is assumed that the gain of the buffer amplifier 7 is substantially equal to the value 1. In this case, the gain and the operating point of the buffer amplifier 6 are to be determined so that the difference is produced between R* and R which generates the output pressure y, which latter value, in turn, is required for X =0.

Assuming that the gain of the buffer amplifier 7 is equal to l, and the transmission factor of the timing elements T and T, has likewise reached the value 1, which is satisfied for t then R y and R y. This means that the output pressure y of the controller is applied to the resistor of the rated value pressure divider and to the input of the buffer amplifier 6. Then, the output pressure of the buffer amplifier 6 is the positive feedback pressure signal R, which is applied to the resistor 3 of the actual value pressure divider.

Starting with the assumption that the flow Q through a throttle resistor is proportional to the pressure difference across the resistor, the following relationships result for the input pressures E, and E wherein A is the ratio of the throttle resistances.

With x w, the relationship becomes:

This, inserted in the equation of the differential amplifier:

Y" V l E2) yu results in the relationship:

R (1+ tv+'1) y 1+X/XV y,

which is to be satisfied by the buffer amplifier 6.

Accordingly, the gain v of this buffer amplifier is to be brought to the value (l+ t/)t V 1). This means that this gain must be varied in correspondence with the adjustable P-range x since A-x,,. Also, the gain v of the differential pressure amplifier l and of the additional amplifier which may be connected thereafter, exerts an influence on the value v of the gain of the buffer amplifier to be adjusted.

FIG. 2 shows an embodiment in accordance with the circuit of FIG. 1. Between the resistors 2 and 3 of the actual value pressure divider, the signal pressure E, is derived, and between the resistors 4 and 5 of the rated value pressure divider, the input pressure E is obtained, which pressures are effective on the input pressure chambers, bounded by elastic walls 8, of the differential pressure amplifier (control amplifier) l. The elastic walls 8 serve for the adjustment of two balls 23 displaceably arranged in a bore of constant diameter in the housing. At the level of contact of the two balls, the pressure supply duct 9 terminates in the bore, through which duct compressed air with the supply pressure p enters the bore. At the spacing of one-half the ball diameter, the output duct 10 is provided, from which the output pressure signal y is derived. in correspondence with the pressure difference E E the balls are shifted and control the feed and discharge of compressed air to and from the output duct 10, to which the outlet pressure line 10a is connected; from the latter, the output pressure y of the controller is derived.

The buffer amplifier 7 forms a structural unit with the timing element T,.. In this connection, the buffer amplifier 7 is fashioned in accordance with the subject matter of German Patent Application P 20 40 l66.5. This proposed construction likewise starts with the abovedescribed differential pressure amplifier provided with negative feedback in order to adjust the gain. The output pressure of this amplifier is applied, via the resistor 13, to the input pressure chamber E, from which compressed air flows against the atmospheric pressure p via the adjustable resistor 14 fashioned as a needle valve. The resistors 13 and 14 constitute the negative feedback pressure divider. With equally large effective surface areas of the diaphragms 8, the gain v of the buffer amplifier 7 is equal to 1, if the ratio A of the resistors 13 and 14 is adjusted to the value A l/V-l, wherein V represents the gain of the differential pressure amplifier, as described in greater detail in German Patent Application P 20 40 166.5. In order to obtain a compact arrangement, the input pressure chamber E of the buffer amplifier 7 is fashioned as the volume 12 of the timing element T,,, and the throttle resistor 11, fashioned as a needle valve, is disposed in the housing of the buffer amplifier.

The output pressure of the buffer amplifier 7 is fed, via the pipeline 15, to the throttle resistor 5 of the rated value pressure divider and to a resistor, fashioned as a needle valve 16, of the timing element T, which forms a structural unit with the buffer amplifier 6. Also the buffer amplifier 6 is fashioned as a differential pressure amplifier, the gain 11 of which is adjustable between 1 v V by means of the negative feedback pressure divider l7, 18 (as explained in greater detail in German Patent Application P 20 40 166.5). The volume 19 of the timing element T is included in the input pressure chamber E, of the buffer amplifier; within this volume 19, the spring 24 is disposed, the bias of which is adjustable by means of the screw 21. The spring 20 exerts a force on the comparison member of the differential pressure amplifier serving to adjust the operating point of the buffer amplifier 6 (as described in German Patent Application P 20 40 166.5). The output pressure of the buffer amplifier 6 is applied, via the pipeline 22, to the resistor 35 of the actual value pressure divider.

FIG. 3 shows a circuit wherein a D-relay 22 is arranged in the actual value branch, so that the timing element T, in the negative feedback channel is eliminated. In this case, the time derivation x of the actual value is present at the resistor 2 of the actual value pressure divider. Due to the elimination of the timing element T,., the buffer amplifier 7 is not required. The circuit contains the resistors 2, 3, 4, and 5, the differential pressure amplifier 1, and the timing element T after which the buffer amplifier 6 is inserted.

FIG. 4 shows a practical embodiment of the circuit according to FIG. 3 and elements common to the arrangement of FIG. 2 are designated with corresponding reference numerals. The D-relay 22 is the subject matter of German Patent Application P 20 40 168.7. The spring bellows 32 define the input pressure chambers of a differential pressure amplifier of a design as described above. These bellows are in operative connection with the balls 23 controlling the feed and discharge of compressed air to the output duct 24. To the duct 24, the resistor 25 is connected, which latter constitutes a pressure divider together with the resistor 26. Between these resistors, compressed air is withdrawn, which latter flows via the adjustable resistor 27 (needle valve) into the input pressure chamber E of the differentiaLpressure amplifier. This chamber serves as the volume 28 of the timing element T,,, the latter being disposed in the negative feedback channel of the amplifier, and determining the time characteristic of the D- relay 22 (as explained in greater detail in German Patent Application P 40 l66.5).

To the actual value pressure divider 2 and 3 the time derivation x and the positive feedback signal R derived from the buffer amplifier 6'are applied. At the setpoint pressure divider 4, S is present the setpoint w and the negative feedback signal R which is equal to the output pressure y of the controller. The pressures E, and E derived from this pressure divider control modulate the differential pressure amplifier 1, after which a second amplifier is connected. The output channel 28 of the first amplifier terminates in the input pressure chamber E of the second amplifier. The latter yields the output pressure y of the controller. The second differential pressure amplifier is, in turn, provided with a negative feedback branch wherein the pressure divider resistors 29 and 30 are arranged. This pressure divider makes it possible to render the degree of amplification of the second amplifier stage adjustable. As in FIG. 2, the buffer amplifier 6 and the timing element T, are also combined into a structural unit in FIG. 4.

FIG. 5 schematically illustrates the above-described structural unitsdisposed on a BASE PLATE provided with connection lines.

What is claimed is:

1. Circuit for pneumatic PlD-controllers comprising at least one control amplifier in the form of a differential pressure amplifier having first and second inputs and producing an output signal dependent on an applied setpoint pressure and on an applied actual value pressure, first and second timing'elements connected in respective feedback branches in series with respective first and second pressure dividers, said first pressure divider applying to said first input of said amplifier a pressure formed from the sum of said actual value pressure and a positive feedback signal, said second pressure divider applying to said second input of said amplifier a pressure produced from the sum of the setpoint pressure and a negative feedback signal, which pressures serve as the input signals of the control amplifier, a first buffer amplifier connected between said first timing element and said first pressure divider and a second buffer amplifier connected between said second timing element and said second pressure divider.

2. Circuit for pneumatic controllers according to claim 1, characterized in that the gain in at least one of the buffer amplifiers deviates from unity.

3. Circuit for pneumatic controllers according to claim 2, characterized in that the gain of at least one of the buffer amplifiers is adjustable.

4. Circuit for pneumatic controllers according to claim 3, characterized in that each of said first and second pressure dividers is formed by a pair of series connected resistors and the gain of said first buffer amplifier equals l+)\ V+ 1, wherein A is the ratio of the resistors of said first and of said second pressure dividers, and V is the gain of said control amplifier.

5. Circuit for pneumatic controllers according to claim 4, characterized in that spring means is provided in at least one of the buffer amplifiers to render the zero operating point thereof adjustable.

6. Circuit for pneumatic controllers according to claim 4, characterized in that each of said buffer ampli- I fiers is fashioned as a differential pressure amplifier, the output pressure of which is fed to a respective input of said control amplifier via a respective one of said first and second pressure dividers. and each buffer amplifier including an additional pressure divider for adjusting the gain of the respective buffer amplifier.

7. Circuit for pneumatic controllers according to claim 6, characterized in that the differential pressure amplifier of each buffer amplifier and that of the control amplifier consists of two elastic walls defining respective input pressure chambers which are in operative connection with two balls of equal diameter displaceably arranged in a housing bore of constant diameter, a feed duct for compressed air being arranged in alignment with the contact point of the balls and at least one output channel being provided which terminates, ata spacing of one-half ball diameter from the feed duct into the housing bore.

8. Circuit for pneumatic controllers according to claim 7, characterized in that one input pressure chamber of said first buffer amplifier serves also as a chamber of said first timing element, the additional pressure divider of said first buffer amplifier including an adjustable throttle resistor of the first timing element disposed in the housing of the first buffer amplifier.

9. Circuit for pneumatic controllers according to claim 7, characterized in that one input pressure chamber of said second buffer amplifier serves also as a chamber of said second timing element, the additional pressure divider of said second buffer amplifier including an adjustable throttle resistor of the second timing element disposed in the housing of the second buffer amplifier.

10. Circuit for pneumatic controllers according to claim 6, characterized in that said control amplifier, as well as the resistors of the first and of the second pressure dividers are disposed in one housing.

11. Circuit for pneumatic controllers according to claim 6, characterized in that said first buffer amplifier and said first timing element form a structural unitdisposed on a base plate provided with pressure connection lines.

12. Circuit for pneumatic controllers according to claim 6, characterized in that said second buffer amplifier and said second timing element form a structural unit disposed on a base plate provided with pressure connection lines.

13. Circuit for pneumatic controllers according to claim 6, characterized in that said control amplifier, as well as the resistors of the first and of the second pressure dividers form a structural unit mounted on a base plate provided with pressure connection lines.

14. Circuit for pneumatic PlD-controllers comprising at least one control amplifier in the form of a differential pressure amplifier having first and second inputs and producing an output signal dependent on an applied setpoint pressure and on an applied actual value pressure, first and second timing elements connected through respective throttle resistors forming a first pressure divider connected to said first input of said control amplifier, a second pressure divider formed by an additional pair of throttle resistors connected to said second input of said control amplifier and comparing the output of said control amplifier to said setpoint value pressure, said first pressure divider comparing the output of said control amplifier to the output of one of said timing elements, said control amplifier being connected between one of said timing elements and said first pressure divider.

15. Circuit for pneumatic controllers according to claim l4 wherein an additional control amplifier is connected in series with the output of said one control amplifier.

16. Circuit for pneumatic controllers according to claim 14, further comprising a buffer amplifier connected at one of a first position between said first timing element and said first pressure divider and a second position between said second timing element and said first pressure divider, characterized in that the gain of said buffer amplifier is adjustable.

17. Circuit for pneumatic controllers according to claim 16, characterized in that the gain of said buffer amplifier equals 1+A/A V 1, wherein A is the ratio of the resistors of s a id first and o f said second pressure di viders, and V is the gain of said control amplifier.

18. Circuit for pneumatic controllers according to claim 17, characterized in that spring means is provided in said buffer amplifier to render the zero operating point thereof adjustable.

19. Circuit for pneumatic controllers according to claim 18, characterized in that said buffer amplifier is fashioned as a differential pressure amplifier, the output pressure of which is fed to said first input of said control amplifier via said first pressure divider, and said buffer amplifier including an additional pressure divider for adjusting the gain thereof.

20. Circuit for pneumatic controllers according to claim 19, characterized in that one input pressure chamber of said buffer amplifier serves also as a chamber of said second timing element, the additional pressure divider of said buffer amplifier including an adjustable throttle resistor of the second timing element disposed in the housing of the buffer amplifier. 

1. Circuit for pneumatic PID-controllers comprising at least one control amplifier in the form of a differential pressure amplifier having first and second inputs and producing an output signal dependent on an applied setpoint pressure and on an applied actual value pressure, first and second timing elements connected in respective feedback branches in series with respective first and second pressure dividers, said first pressure divider applying to said first input of said amplifier a pressure formed from the sum of said actual value pressure and a positive feedback signal, said second pressure divider applying to said second input of said amplifier a pressure produced from the sum of the setpoint pressure and a negative feedback signal, which pressures serve as the input signals of the control amplifier, a first buffer amplifier connected between said first timing element and said first pressure divider and a second buffer amplifier connected between said second timing element and said second pressure divider.
 2. Circuit for pneumatic controllers according to claim 1, characterized in that the gain in at least one of the buffer amplifiers deviates from unity.
 3. Circuit for pneumatic controllers according to claim 2, characterized in that the gain of at least one of the buffer amplifiers is adjustable.
 4. Circuit for pneumatic controllers according to claim 3, characterized in that each of said first and second pressure dividers is formed by a pair of series connected resistors and the gain of said first buffer amplifier equals 1+ lambda / lambda V + 1, wherein lambda is the ratio of the resistors of said first and of said second pressure dividers, and V is the gain of said control amplifier.
 5. Circuit for pneumatic controllers according to claim 4, characterized in that spring means is provided in at least one of the buffer amplifiers to render the zero operating point thereof adjustable.
 6. Circuit for pneumatic controllers according to claim 4, characterized in that each of said buffer amplifiers is fashioned as a differential pressure amplifier, the output pressure of which is fed to a respective input of said control amplifier via a respective one of said first and second pressure dividers, and each buffer amplifier including an additional pressure divider for adjusting the gain of the respective buffer amplifier.
 7. Circuit for pneumatic controllers according to claim 6, characterized in that the differential pressure amplifier of each buffer amplifier and that of the control amplifier consists of two elastic walls defining respective input pressure chambers which are in operative connection with two balls of equal diameter displaceably arranged in a housing bore of constant diameter, a feed duct for compressed air being arranged in alignment with the contact point of the balls and at least one output channel being provided which terminates, at a spacing of one-half ball diameter from the feed duct into the housing bore.
 8. Circuit for pneumatic controllers according to claim 7, characterized in that one input pressure chamber of said first buffer amplifier serves also as a chamber of said first timing element, the additional pressure divider of said first buffer amplifier including an adjustable throttle resistor of the first timing element disposed in the housing of the first buffer amplifier.
 9. Circuit for pneumatic controllers according to claim 7, characterized in that one input pressure chamber of said second buffer amplifier serves also as a chamber of said second timing element, the additional pressure divider of said second buffer amplifier including an adjustable throttle resistor of the second timing element disposed in the housing of the second buffer amplifier.
 10. Circuit for pneumatic controllers according to claim 6, characterized in that said control amplifier, as well as the resistors of the first and of the second pressure dividers are disposed in one housing.
 11. Circuit for pneumatic controllers according to claim 6, characterized in that said first buffer amplifier and said first timing element form a structural unit disposed on a base plate provided with pressure connection lines.
 12. Circuit for pneumatic controllers according to claim 6, characterized in that said second buffer amplifier and said second timing element form a structural unit disposed on a base plate provided with pressure connection lines.
 13. Circuit for pneumatic controllers according to claim 6, characterized in that said control amplifier, as well as the resistors of the first and of the second pressure dividers form a structural unit mounted on a base plate provided with pressure connection lines.
 14. Circuit for pneumatic PID-controllers comprising at least one control amplifier in the form of a differential pressure amplifier having first and second inputs and producing an output signal dependent on an applied setpoint pressure and on an applied actual value pressure, first and second timing elements connected through respective throttle resistors forming a first pressure divider connected to said first input of said control amplifier, a second pressure divider formed by an additional pair of throttle resistors connected to said second input of said control amplifier and comparing the output of said control amplifier to said setpoint value pressure, said first pressure divider comparing the output of said control amplifier to the output of one of said timing elements, said control amplifier being connected between one of said timing elements and said first pressure divider.
 15. Circuit for pneumatic controllers according to claim 14 wherein an additional control amplifier is connected in series with the output of said one control amplifier.
 16. Circuit for pneumatic controllers according to claim 14, further comprising a buffer amplifier connected at one of a first position between said first timing element and said first pressure divider and a second position between said second timing element and said first pressure divider, characterized in that the gain of said buffer amplifier is adjustable.
 17. Circuit for pneumatic controllers according to claim 16, characterized in that the gain of said buffer amplifier equals 1+ lambda / lambda V + 1, wherein lambda is the ratio of the resistors of said first and of said second pressure dividers, and V is the gain of said control amplifier.
 18. Circuit for pneumatic controllers according to claim 17, characterized in that spring means is provided in said buffer amplifier to render the zero operating point thereof adjustable.
 19. Circuit for pneumatic controllers according to claim 18, characterized in that said buffer amplifier is fashioned as a differential pressure amplifier, the output pressure of which is fed to said first input of said control amplifier via said first pressure divider, and said buffer amplifier including an additional pressure divider for adjusting the gain thereof.
 20. Circuit for pneumatic controllers according to claim 19, characterized in that one input pressure chamber of said buffer amplifier serves also as a chamber of said second timing element, the additional pressure divider of said buffer amplifier including an adjustable throttle resistor of the second timing element disposed in the housing of the buffer amplifier. 